1. Field of the Invention
The invention relates generally to, liquid-cooling systems for high power dissipating electronic components mounted on printed circuit boards and more specifically a cost effective, high performance, high reliable cold plate.
2. Description of Related Art
Electronic circuit board assemblies tend to generate varying levels of heat to be dissipated during operation. If left unchecked, component overheating may affect performance or even failure of the electronic components. For relatively low-power systems, air cooling and heat sink techniques often adequately maintain lower operating temperatures to such electronic components. Application of printed circuit boards that employ high power electronic components demanded by such equipment used, often require liquid cooling to minimize the cooling system size, and heat transfer medium required transmitting larger amount of heat rate using relatively smaller size cooling system.
Several different liquid-cooling methods have been proposed in the field of cooling high power dissipating electronic components mounted on printed circuit boards. One of the more popular liquid-cooling mechanisms employs an aluminum cover assembly that mounts to a circuit board in overlaying close-fitting relationship to the surface-mounted electronics. This kind of cooling apparatus is commonly referred to as a cold plate.
Conventional cold plates typically comprise a relatively flat thermally conductive body formed with an engagement surface that closely mirrors the surface configuration or topology of the circuit board. An internal cooling channel is formed in the plate to circulate cooling fluid through the body and draw heat away from the cold plate during operation. The plate mounts to the circuit board with the respective electronic components nested in corresponding close-fitting recesses.
While conventional cold plates offer significant cooling advantages for printed circuit boards, as compared to air-cooled systems, some of the drawbacks involve cost and reliability. Typically, the costs associated with cold plates often reflect long lead times and complex manufacturing operations, which most often may lead to lower reliability. Consequently the expense to employ a traditional cold plate system coupled with reliability issues in a printed circuit board environment is often undesirably high cost and lower reliability.
In an effort to address these problems, those skilled in the art have advanced many proposals for design and manufacturing cold plates reflecting the generally structure described above. One method disclosed in U.S. Pat. No. 4,196,775. Involving selecting a thin tube, and casting the plate around the tube to construct the internal fluid channel. A variant on this technique utilizes a plurality of parallel tubes to improve the surface area of the coolant-to-plate interface. While this xe2x80x9ctubexe2x80x9d technique works well for its intended low-flow applications, the limited surface area at the coolant-to-plate interface generally restricts the cooling capability to relatively lower heat dissipation levels.
Another conventional method of design and manufacturing cold plates omits the tubes, employs the steps of machining or casting two oppositely confronting halves of a cold plate. The confronting surfaces of each cold plate half are each respectively formed with complemented inwardly opening cavities, that, when brought together, form a complete fluid channel. The two halves are then brought together and vacuum brazed to form an integral cold plate unit.
While this method works well for its intended applications, the costs involved in machining the two halves, and carrying out the relatively complex vacuum brazing step are often undesirably high. Moreover, the vacuum brazing step generally minimizes the cold plate production throughout because of the low allowable tolerances involved when carrying out the vacuum brazing process.
What is needed and has been heretofore unavailable is a high-performance, cost-effective cold plate configuration that lends itself to a high level of manufacturability, and a method that implements straightforward design and fabrication steps to minimize costs and production delays, which in turn simplifies the design of the cooling system, and its components. The cold plate and method of the present invention satisfies these needs.
The prior art teaches the use of heat dissipation devices for maintaining temperature in electrical and electronic circuits, but does not teach such a device having the features of high performance, low cost and ease of manufacture. The present invention fulfills these needs and provides further related advantages as described in the following summary.
The apparatus and method of this invention provide cost-effective, high performance and modular way or producing cold plate configuration that is capable of being produced and assembled through putting together three modules. The three modules are produced by mass production techniques and are ready to form the cold plate assembly. Substantial costs and delays in fabrication are minimized in addition to increase in reliability, which lower the breakdown risk level or increase the mean time between failure (MTBF).
To realize the foregoing advantages, the invention in one form comprises a cold plate assembly for cooling heat sources on a printed circuit board, i.e., the high power dissipating electronic components. The cold plate assembly comprises three modules including: a thermally conductive base having an outer surface for thermal interface with a heat source, a heat pipe thermal plane or individual heat pipes to transmit the heat from the heat source with minimum thermal gradient, and one or two compact heat exchangers of laminated or finned construction, depending on the application; where the heat is transferred to a cooling fluid for removal.
The invention comprises a method of fabricating a cold plate assembly that achieves a greater homogeneity of temperature through the printed circuit board in order to avoid any localized heating which might adversely affect the electronic components. Consequently, the invention likewise has as one object, the provision of a printed circuit board with improved thermal characteristics. This allow it to dissipate a greater flow of heat in the direction of the one or two heat exchangers mounted on the farther ends of the cold plate base with minimum thermal gradient. Printed circuit board assemblies require increased thermal capability needed by new technology. Using this cold plate for cooling, printed circuit board assemblies can employ increasing power or heat flux density from its components and maintain component junction temperature within specification limits. Also this invention realizes easy way to production of cold plates and put into service. The invention also improve the reliability of the cold plate, since none of the elements that make the cold plate have moving elements, they are extremely reliable.
The advantages of this invention are all a direct result of using first, the heat pipe thermal plane or individual heat pipes. The heat pipe technology allows heat transfer with an extremely high and effective thermal conductivity. As a passive device, heat pipe thermal plane is a plate utilizing embedded copper/water heat pipes to carry the heat from components to a liquid or air cooled cold wall. While cooling at both edges in recommended for maximum plate performance, single edge cooling is possible for derated performance. The heat pipe thermal plate operation is insensitive to mounting orientation. The heat pipe is a heat transfer device with extremely high effective thermal conductivity. Heat pipes are evacuated vessels, typically circular in cross sections, which are back-filled with a small quantity of a working fluid. They are totally passive and are used to transfer heat from a heat source (electronic components) to a heat sink (heat exchanger) with minimal temperature gradients. They are also used to isothermalize surfaces. Heat pipes transfer heat by the evaporation and condensation of a working fluid. As heat is input at the evaporator, fluid is vaporized, creating a pressure gradient in the pipe. This pressure gradient forces the vapor to flow along the pipe to the cooler section where it condenses, giving up its latent heat of vaporization. The working fluid is then returned to the evaporator by capillary forces developed in the porous wick structure or by gravity. The heat pipe thermal plane or individual heat pipes used in this invention utilizes working fluid range from ammonia, water, acetone and methanol. The compact heat exchanger is efficient with high heat transfer density.
The method of fabricating the cold plate includes the steps of: 1) selecting a plate of thermally conductive material, 2) selecting a heat pipe thermal plane or individual heat pipes that meets the dimensions and heat transfer capacity needed to cool the printed circuit board electronic components, and 3) selecting the type and size of the one or two laminated or finned compact heat exchangers depending on the heat capacity to be dissipated. The bonding of the three modules is completed as shown in the figures.